Allow anonymous users to subscribe to the newsletter API (bug 1046888)

https://bugzilla.mozilla.org/show_bug.cgi?id=1046888

For about:apps we are going to let anonymous users subscribe to the newsletter. Since the API already required an email to be passed (without forcing it to be the same as the email on the current user account) we just need to change the permission class.
`CreateAPIViewWithoutModel` defaults to `permission_classes = (AllowAny,)`